1. Field of the Invention
The present invention relates to a mandrel mill capable of preventing stripping miss which can take place during the production of a seamless tubing.
2. Description of the Related Art
A method of producing a seamless tubing comprises piercing a heated billet with a piercer, and rolling the inner surface of the pierced material with a mandrel mill, which is followed by finish rolling.
A mandrel mill employed in such a rolling process generally includes, as shown in FIG. 3, a plurality (usually 5 to 8) of roll stands 1, each having a plurality of pairs of grooved rolls 2 and 2'. The plurality of roll stands 1 are serially arranged with the axes of adjacent roll pairs extending perpendicular to each other, thereby defining a serial arrangement of the grooves of the rolls. A mandrel bar 3 is disposed in and extended through the serial arrangement of the grooves. The mandrel bar 3 rolls the inner surface of a tubing material 4.
When rolling is being performed with such a mandrel mill, the inner surface of the tubing material may be brought into tight contact with the mandrel bar. After the rolling, the mandrel bar and the tubing material may be stuck together, making it impossible to withdraw the mandrel from the tubing material. Such a phenomenon is called a "striping miss".
A stripping miss is more likely to occur when the tubing is made of a high-alloy steel than when it is made of an ordinary carbon steel. A high-alloy steel has a relatively great coefficient of thermal expansion. In the former case, therefore, the tubing material has a relatively great heat shrinkage, and it relatively easily engages in tight contact with the mandrel bar. In addition, the tubing material has a relatively great deformation resistance, and it exerts a relatively great force with which the tubing material, in tight contact with the mandrel bar, fastens onto the mandrel bar. Thus, a stripping miss might be expected to occur when dealing with a high-alloy steel.
Once a stripping miss occurs, the operation of the rolling line must be suspended. The tubing material with the mandrel bar stuck therein is taken out of the line, and, in order to separate the tubing material from the mandrel bar, the joint between them has to be melted away with acetylene gas flame or the like. The separated tubing material becomes scrap. On the other hand, the mandrel bar cannot be used until the separating operation is completed. Thus, a stripping miss can seriously trouble the continuing operation of a mandrel mill.
The above-described problem of a mandrel bar may similarly arise in the case of a retained mandrel mill in which, during rolling, the rear end of the mandrel bar is retained in such a manner that movement of the mandrel bar is forcibly controlled at a certain fixed speed lower than the speed of the material at the exit of the mill.
Various methods have been proposed with a view to preventing scratch-formation on the inner surface of the tubing material or preventing stripping miss.
One of the most generally-known methods comprises adjusting the speed of rotation of the rolls of adjacent stands to adjust the stress applied to the parts of the tubing material between adjacent stands, so as to control the cross-sectional configuration of the tubing material. For instance, "Basic Load Characteristics and Deformation Characteristics" (on pages 545 to 548 of "Theses of 1984 Spring Meeting on Plastic Working") shows with regard to two-stand continuous rolling, the art of changing the speed of rotation of the rolls of the first stand so as to control the tensile force between the first and the second stands as well as the outer diameter (width) of the tubing material at the exit of the second stand.
Japanese Patent Laid-open No. 60-46805 proposes the art of effecting an appropriate rolling reduction at the final stand so as to form a relief portion in the roll grooves of the final stand, the thus formed clearance between the mandrel bar and the inner surface of the tubing material enabling an easy drawing of the mandrel bar.
Japanese Patent Publication No. 59-24885 proposes the art of disposing a forming roll, which may be either a driven or non-driven type, between adjacent stands of a mandrel mill, and causing an edge portion of the tubing material projecting from the previous stand to be gripped by the forming roll, so that an appropriate clearance is provided between the inner surface of the tubing material and the mandrel bar.
With the method shown in the above-identified thesis, although it is possible to control the configuration of a central portion of the tubing material which can be held simultaneously by a plurality of stands, it is not possible to control the configuration of the forward and rearward end portions of the tubing material which cannot be subjected to a sufficient compression force between a plurality of stands. It is generally known that the forward and rearward end portions of a tubing material tend to be in an under fill condition wherein the entire inner circumference of the material contacts the mandrel bar.
With the method proposed in Japanese Patent Laid-open No. 60-46805, if the entire inner circumference of the tubing material at the entrance of the final stand contacts the mandrel bar, it is not possible to form an appropriate clearance between the mandrel bar and the inner surface of the tubing material regardless of how the rolling reduction at the final stand is adjusted or how a relief portion is formed in the roll grooves of the final stand.
The method proposed in Japanese Patent Publication No. 59-24885 is effective when the tubing material at the exit of the previous stand has a projecting edge portion. However, when the tubing material is in contact with the mandrel bar throughout the circumference thereof and simultaneously has no projecting edge portion, gripping with a forming roll does not make it possible to provide an appropriate clearance between the inner surface of the tubing material and the mandrel bar.
Thus, none of the above-described art is able to form an appropriate clearance between the inner surface of the tubing material and the mandrel bar when the entire inner circumference of the rearward end portion of the tubing material contacts the mandrel bar.